In recent years, ultra super critical boilers in which steam temperature and pressure are to increase for high efficiency have been newly built in the world. Specifically, it is planned to increase the steam temperature which is heretofore approximately 600° C. up to 650° C. or more, or further up to 700° C. or more, and to increase the steam pressure which is heretofore approximately 25 MPa up to approximately 35 MPa. The reason for the above is based on the fact that energy saving, efficient use of resources, and reduction in CO2 emission for environmental protection are one of objects for solving energy problems and are important industrial policies. In addition, in a case of boilers for power generating plants and reacting furnaces for chemical industrial plants where fossil fuel is combusted, it is advantageous to use high efficient ultra super critical boilers and high efficient reacting furnaces.
With increasing the steam temperature and pressure, the temperature of plates, forgings, or the like which are used as superheater tubes in boilers, chemical industrial reaction tubes, and heat resisting and pressure resisting materials increases up to 700° C. or more during actual operation. Thus, it is required for the alloy used in the above severe environment for a long time to be excellent in not only high temperature strength and high temperature corrosion resistance but also creep rupture ductility or the like.
Furthermore, at the time of maintenance such as repairs after usage for a long time, it is necessary for materials aged by the usage for the long time to be subject to the treatment such as cutting, working, or welding. Thus, it has been eagerly required to have not only characteristics as new materials but also soundness as aged materials. In particular, it has been required to be excellent in reheat cracking resistance in order to make the welding possible after the usage for the long time.
With regard to the above severe requirements, in the conventional austenitic stainless steels or the like, creep rupture strength (creep rupture time) is insufficient. Thus, it is necessary to use a Ni-based heat resistant alloy in which precipitation strengthening derived from intermetallic compounds such as γ′ phase is utilized. Herein, the creep rupture strength represents an estimated value obtained by Larson-Miller parameter using a creep test temperature and a creep rupture time. Specifically, the estimated value of creep rupture strength increases with an increase in the creep rupture time. Thus, in the present invention, the creep rupture time is used as a parameter of high temperature strength.
Patent Documents 1 to 9 disclose Ni-based alloys used in the severe environment such as high-temperature as described above. In the Ni-based alloys, solid solution strengthening is utilized by containing Mo and/or W, and precipitation strengthening derived from intermetallic compounds such as γ′ phase, specifically Ni3(Al, Ti), is utilized by containing Al and Ti.
Among the Patent Documents, the alloys disclosed in the Patent Documents 4 to 6 include 28% or more of Cr, so that a large number of α-Cr phase having a bcc (body centered cubic) structure precipitates, which contributes to the strengthening.